This section is intended to provide a background to the various embodiments of the invention that are described in this disclosure. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
Communication devices such as User Equipments (UE) are also known as terminals, user terminals (UTs), wireless terminals, wireless transmit/receive units (WTRUs), etc. UEs are generally enabled to communicate wirelessly in a radio network, which may sometimes also be referred to as e.g. cellular communication network, wireless communication system, cellular radio system or cellular network. The communication may be performed e.g. between two UEs, between a user equipment and a regular telephone and/or between a UE and a server via a Radio Access Network (RAN) and possibly one or more core networks (CN), comprised within the cellular communications network.
UEs may further be referred to as mobile telephones, cellular telephones, laptops, or tablet computers with wireless capability, just to mention some further examples. The UEs in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another UE or a server. The term UE as used herein also comprises Machine Type Communication (MTC) devices, i.e. devices with communication capability of machine-type. Examples of MTC devices include e.g. devices such as sensors, measurement devices etc that do not necessarily involve any interaction with a user. More information on MTC use case scenarios and examples of MTC devices can be found e.g. in the Technical Specification 3GPP TS 22.368 V12.1.0.
The radio network generally covers a geographical area which is divided into cell areas, wherein each cell area is served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “base station”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and/or cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site.
LTE Mobility
Mobility management may be a challenging task in cellular communications systems and a well functioning mobility performance may therefore be important to the quality experienced by the end user. The Radio Resource Control protocol, RRC, see e.g. the Technical Specification 3GPP TS 36.331 V11.2.0, is the main signaling protocol for configuring, re-configuring and general connection handling in the Long Term Evolution (LTE) radio access network (E-UTRAN). RRC controls many functions such as connection setup, mobility, measurements, radio link failure and connection recovery.
A UE in LTE can be in two RRC states: RRC_CONNECTED and RRC_IDLE. In RRC_CONNECTED state, mobility is generally network-controlled based on e.g. measurements provided by the UE. That is, the network decides when and to which cell a UE should be handed over, based on e.g. measurement reports provided by the user equipment. The network, i.e. the LTE radio base station which is called evolved NodeB (eNB) in E-UTRAN, configures various measurement events, thresholds etc based on which the UE then sends reports to the network, such that the network can make a decision to hand over the UE to a better cell as the UE moves away from its present cell.
A detailed illustration of a LTE RRC handover procedure can be found in section 10.1.2 of the Technical Specification 3GPP TS 36.300 V11.4.0, see e.g. FIG. 10.1.2.1-1. FIG. 1 illustrates a simplified signaling scheme of the LTE handover, HO, procedure presented therein. It should be noted that the HO command shown in FIG. 1 is in fact prepared in the Target eNB, i.e. the eNodeB that the UE will be handed over to, but the message is transmitted via the Source eNB. That is, from the UE's perspective the message comes from the Source eNB (although it originates from the Target eNB).
In RRC_IDLE, mobility is generally handled by UE-based cell-selection, where a “nomadic” UE selects the best cell to camp on, based e.g. on various specified criteria and parameters that are broadcasted in the cells. For example, various cells or frequency layers could be prioritized over other cells or frequency layers, such that the UE tries to camp on a particular cell as long as the measured quality of a beacon or pilot in that cell is a threshold better than some other beacon or pilot received from other cells.
The present disclosure is primarily focusing on challenges associated with network-controlled mobility as described above, i.e. for an UE in the RRC_CONNECTED state. Some of the challenges associated with failing handovers are therefore described in further detail below:
In a regular situation, and when a UE in RRC_CONNECTED state is moving out from the coverage of a first cell, also called source cell, it should be handed over to a neighboring cell, also called target cell or second cell before loosing the connection to the first cell. In other words, it is desirable that the connection is maintained with no or minimal disruption throughout the handover, such that the end-user is unaware of the ongoing handover. In order to succeed with this, it is important that:                the measurement report that indicates the need for mobility is transmitted by the UE and received by the Source eNB, and        the Source eNB has sufficient time to prepare the handover to the target cell (by, among other things, requesting a handover from the Target eNB controlling the target cell), and        the UE receives the handover command message from the network, as prepared by the Target eNB in control of the target cell and sent via the source cell to the user equipment, see FIG. 1.        
In addition, and in order for the handover to be successful, the user equipment must finally succeed in establishing a connection to the target cell, which in LTE generally requires a successful random access request in the target cell, and a subsequent transmission of a HO complete message from the UE to the Target eNB.
Thus, in order for the handover to succeed, it may be important that the sequence of events leading to a successful handover is started sufficiently early, so that the radio link to the first cell over which this signaling takes place does not deteriorate too much before completion of the signaling. If such deterioration happens before the handover signaling is completed in the source cell (i.e. first cell), then the handover is likely to fail. Such handover failures are clearly not desirable. The current RRC specification, i.e. 3GPP TS 36.331 V11.2.0, therefore provides various triggers, timers, and thresholds in order to adequately configure measurements, such that the need for handovers can be detected reliably, and sufficiently early.
In FIG. 1, the exemplified measurement report is generally triggered by a measurement event called A3 event, which in short means that a neighbor cell is found to be an offset better than the current serving cell. This means that a measurement report is sent to the network when a criterion or criteria associated with the event is satisfied. There exists many different measurement event types, and it should be noted that there are multiple events that could be configured to trigger a measurement report.
Radio Link Failure (RLF) and RRC Connection Re-Establishment
It may occur that a UE looses coverage to the cell that the UE is currently connected to. This could occur in a situation when a UE enters a fading dip, or that a handover was needed as described above, but the handover failed for one or another reason. This may be particularly true if the “handover region” is very short, as will be further described below.
The quality of the radio link is typically monitored in the user equipment e.g. on the physical layer, as described in the technical specifications 3GPP TS 36.300 V11.4.0 (see e.g. section 10.1.6), 3GPP TS 36.331 V11.2.0 (see e.g. section 5.3.11), and 3GPP TS 36.133 V11.3.0 (see e.g. Annex A.7.3), and summarized below.
Upon detection that the physical layer experiences problems according to criteria defined in e.g. 3GPP TS 36.133 (see e.g. Annex A.7.3), the physical layer can send an indication to the RRC protocol of the detected problems called out-of-sync indication. After a configurable number, N310, of such consecutive indications, a timer, T310, is started. If the link quality is not improved (i.e. recovered) while T310 is running, i.e. there are no N311 consecutive “in-sync” indications from the physical layer, a radio link failure, RLF, is declared in the UE, see FIG. 2.
The currently relevant timers and counters described above are listed in FIG. 3 for reference. The UE may e.g. read the timer values and counter values from system information broadcasted in the cell. Alternatively, it is possible to configure the UE with UE-specific values of the timers and counter values using dedicated signaling, e.g. where specific values are provided to specific user equipments with messages directed only to one or more specific UEs.
The function of the timers and counters used for monitoring radio link failure in LTE is presented in the tables of FIG. 3.
If T310 expires, the UE initiates a connection re-establishment to recover the ongoing RRC connection. This procedure includes cell selection by the UE. That is, the RRC_CONNECTED user equipment shall try to autonomously find a better cell to connect to, since the connection to the previous cell failed according to the described measurements. It could occur that the UE returns to the first cell anyway, but the same procedure is also then executed. Once a suitable cell is selected as further described e.g. in the technical specification 3GPP TS 36.304 V11.2.0 (see e.g. section 5.2), the UE requests to re-establish the connection in the selected cell. To this end, it is important to note the difference in mobility behavior as an RLF results in UE-based cell selection, in contrast to the normally applied network-controlled mobility.
If the re-establishment is successful, which depends on, among other things, if the selected cell and the eNB controlling that cell was prepared to maintain the connection to the user equipment, which implies that is was prepared to accept the re-establishment request, then the connection between the user equipment and the eNB can resume. In LTE, a re-establishment procedure generally includes a random-access request in the selected cell, followed by higher layer signaling where the user equipment sends a message with content based on which the UE can be identified and authenticated. This may be needed so that the network can trust that it knows exactly which UE is attempting to perform the re-establishment.
A failure of a re-establishment means that the UE goes to the RRC_IDLE state and the connection is generally released. To continue communication, a new RRC connection then has to be requested, and established. A failure could occur e.g. if the eNB that receives the re-establishment request is not able to identify the UE that requests the re-establishment. Such a condition may occur if the receiving eNB has not been informed or prepared for a possible re-establishment from this particular UE.
The reason for introducing the timers T310 and counters N310 described above is to add some freedom and hysteresis for configuring the criteria for when a radio link should be considered as failed and need to be re-established. This is desirable, since otherwise it would affect the end-user performance negatively if a connection was abandoned prematurely if it turned out that the loss of link quality was temporary and the UE succeeded in recovering the connection without any further actions or procedures, e.g. before T310 expires, or before the counter reaches value N310.